CO Reduction to Ethylene and Cyclopropane via a Trappable Ruthenium Methylidene

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Abstract
Ruthenium complexes based on cis-[Ru(bpy)2(CO)2]2+ (bpy is 2,2'-bipyridine) can reduce CO2 and CO to C1 products including methanol, but access to products containing C-C bonds has been elusive. A reaction pathway to convert CO into multicarbon products ethylene and cyclopropane is presented here, along with mechanistic studies elucidating the key intermediates in C-C bond formation. The ruthenium hydroxymethyl complex [Ru(bpy')2(CO)(CH2OH)]+ (bpy' = 5,5'-dimethyl-2,2'-bipyridine) undergoes protonolysis to generate the ethylene complex [Ru(bpy')2(CO)(C2H4)]2+ even at -80 °C, with free ethylene released upon warming to room temperature. Experimental evidence implicates a highly electrophilic methylidene complex [Ru(bpy')2(CO)(CH2)]2+ as the key intermediate. The methylidene was successfully trapped with nitriles and pyridine, forming adducts (ylide complexes) that each have a unique reactivity profile. With an appropriate nitrile, the adduct can be characterized at low temperature before warming generates ethylene. A more stable pyridine adduct [Ru(bpy')2(CO)(CH2pyridine)]2+ was crystallographically characterized. Even ethylene itself is sufficiently nucleophilic to react with the electrophilic methylidene, revealing a route from CO to the C3 hydrocarbon cyclopropane. The methods for controlling the reactivity of hydroxymethyl and methylidene complexes toward C-C bond formation can inform the development of CO and CO2 reduction catalysts.
Funding Information
  • Division of Chemistry (CHE-1828183)
  • University of Central Arkansas
  • Basic Energy Sciences (DE-SC0001011, DE-SC0021173)
  • North Carolina Alliance to Create Opportunity (0450099)
  • UCA ATOM Scholars Program (1930026)

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